1. Field of the Invention
This invention relates to mobile radiotelephone systems, and, more specifically, it relates to means for automatically obtaining an available channel for placing a call from the mobile radiotelephone unit.
2. Description of the Prior Art
In conventional mobile radiotelephone systems there is a problem in respect of the relatively small number of channels available for use. In order to initiate a call, the mobile radiotelephone unit must obtain an open channel. Channel allocation in such a system is generally controlled by a stationary base computer which selects an open channel as the next one to be employed and announces this selection by broadcasting an idle tone on the chosen channel. The base computer continues to broadcast the idle tone on the selected channel until it receives a request for a channel either from a mobile radiotelephone unit or from a standard telephone attempting to call a mobile radiotelephone unit. When such a request is received, the base computer processes the call and also discontinues its broadcast of the idle tone on a previously selected channel. It then selects another unused channel, if one is available, and begins to broadcast the idle tone on the newly chosen channel.
In the normal situation, when a mobile radiotelephone station is turned on, it begins scanning the channels. Upon finding an available or idle channel, by locating the idle tone, the supervisory logic portion of the mobile radiotelephone inhibits further scanning, thereby leaving the mobile station seized on the idle channel. Due to the relatively small number of channels, frequently, during peak hours, all of the available channels are busy. As there are no unused channels, the base station is not broadcasting the idle tone on any channel. This causes all of the operating mobile radiotelephones in the region to continuously scan all channels searching for the idle tone. If the mobile radiotelephone user lifts the handset to make a call while the supervisory logic is scanning for an idle tone under these circumstances, no call can go through, and the supervisory logic informs the user of this fact by illuminating the busy lamp. The user must then hang up and try again. As a result, in this fashion, the user must repeatedly remove the handset and replace it in order to attempt to obtain an available channel. Not only is such an approach to finding an available channel inefficient, but it is also quite disruptive and annoying for an individual to have to repeatedly remove and replace the handset. This inconvenience is enhanced when the mobile unit is in motion, and the individual is attempting to both drive the vehicle and place the call simultaneously.
Numerous prior art mobile radiotelephone systems have been known. In general, the systems are rather complex involving extensive use of complicated computer logic circuits. See, for example, U.S. Pat. Nos. 3,555,424; 3,634,627; 3,764,915; 3,806,804 and 3,894,194. U.S. Pat. No. 2,803,744 discloses multiple unit mobile radiotelephone systems wherein the possibility of manual selection of channels which are not busy is disclosed as well as the possibility of overriding the manual selection in order to permit reception over an assigned channel. U.S. Pat. No. 3,513,264 relates to satellite communication systems and the central coordination of channel assignment.
There remains, therefore, a substantial need for a practical and economical means for eliminating the need to repeatedly remove a mobile radiotelephone handset and replace the same during the period when all channels are busy in order to determine whether one of the busy channels has become available. There remains a further need to provide such a system which is adapted for use with conventional types of mobile radiotelephone systems. Also, there is the need to provide the user with an indication that an available channel has been obtained without requiring the undesirable and burdensome cyclic removal and replacement of the handset.